Jean Pierre Bergmann

Solid-state welding of aluminum to copper—case studies

This paper describes procedures for the joining of aluminum with copper via solid-state welding. Welding of dissimilar materials in order to reach a high strength and durable joints is a reasonable and outstanding challenge for industrial and especially for automotive applications. Nowadays, wiring and electrical systems, as for example batteries, turn out to be main drivers for new developments in the field of dissimilar joining technologies. Joining of aluminum to copper through a melting process leads to brittle intermetallic compounds, which cause failure of the joint already during cool down. Solid-state welding technologies allow the welding below melting temperature, so that phenomena at the interface, which lead to the formation of intermetallics under different conditions, are of interest to the resulting joint properties. Not only mechanical properties, but electrical resistivity and heat conduction are strongly dependent on those effects. Diffusion welding is performed as a model welding procedure, while friction stir welding and hybrid friction diffusion bonding are investigated as technological welding processes in this paper.

Friction Stir Welding of Tailored Blanks of Aluminum and Magnesium Alloys

The following paper describes a feasibility study of butt joining friction stir welding between aluminum alloy AA6016 and magnesium alloys AZ91 and AM50. Because of the variety of inimitable properties according to lightweight design and constructions, the interest in aluminum and magnesium alloys is increasing in many fields of industry. Due to the low solubility of aluminum in magnesium and inverse, these alloys tend to the formation of intermetallic phases during the joining process. This leads to an increasing micro hardness within the seam, which should be avoided. By the use of joining methods with low process temperatures, the formation of intermetallic phases is reduced. According to this circumstance, friction stir welding is an excellent alternative to fusion welding techniques used to join this alloys. The main welding process variables were exposed in the studies of similar butt joints of Al/Al and Mg/Mg. These were examined in connection to their transferability to the dissimilar joints and tailored blanks. Furthermore, the influence of different tool geometry on seam quality was investigated. The effect of process variables (mainly welding speed and revolution speed) were correlated to the results of tensile strength test. The welded samples were assayed in the presence of intermetallic phases.

Experimental investigations and modeling of the melting layer in polymer-metal hybrid structures

The melting layer in thermal joining of polymer-metal hybrid joints is analyzed according to process management and microstructure. These results were transferred to spot joints for describing the material behavior and the resulting joining zone. This leads to a generally valid model concept for laser-based polymer-metal joining.

Remote micro welding with multi-mode and single-mode fiber lasers – A comparison

Both multi-mode and single-mode lasers are well established in welding applications. In micro welding, single-mode lasers are often used while multi-mode lasers are more common when it comes to welding tasks in the macro range.In this work, a 500 W multi-mode and a 1,000 W single-mode fiber laser were compared concerning their practicability in terms of micro welding. For this reason, stainless steel foils in thicknesses of 25 µm and 50 µm were overlap welded with focal diameters between 22 µm and 204 µm using 2D scanning systems. The process boundaries were described and process behavior was determined by examining welding regime, melt flow-induced seam imperfections, and specific energy demand while welding.Additionally, measurements of hardness and tensile tests illustrate usage properties and constraints of both fiber laser concepts in micro welding.Both multi-mode and single-mode lasers are well established in welding applications. In micro welding, single-mode lasers are often used while multi-mode lasers are more common when it comes to welding tasks in the macro range.In this work, a 500 W multi-mode and a 1,000 W single-mode fiber laser were compared concerning their practicability in terms of micro welding. For this reason, stainless steel foils in thicknesses of 25 µm and 50 µm were overlap welded with focal diameters between 22 µm and 204 µm using 2D scanning systems. The process boundaries were described and process behavior was determined by examining welding regime, melt flow-induced seam imperfections, and specific energy demand while welding.Additionally, measurements of hardness and tensile tests illustrate usage properties and constraints of both fiber laser concepts in micro welding.

An innovative joining strategy in order to join zinc coated steels with minimized damaging of the coating

Metallic components undergo during complete lifetime complex loading conditions, which are a combination of mechanical, environmental as well as corrosive loading. The developments within the ULSAB-project allowed the enhancing of mechanical properties of steels, as for example for DP-and CP steel sorts. The corrosion resistance of high strength steels and of deep drawing steel can be only improved through coating technology, as for example through hot dipping in zinc or electrolytic zinc coating. Thermal joining of zinc coated steel sheets presents some difficulties, as the boiling temperature of zinc (907°C) is lower than the steel melting point. As a result of the eruption like evaporation of zinc, pores in the weld seam are present after solidification and the zinc coating near the weldment is spoilt and has to be reestablished. Furthermore the degassing of zinc influences negatively process conditions, as for example arc turbulences when MSG welding and wear of the electrons when resistance spot welding is performed. Even if solutions were developed, brazing of zinc coated steels with CuSi-alloys is nowadays mostly set when joining zinc coated steels. Nevertheless the high melting point of these filler materials (over 900°C) requires very restricted process strategies and damaging of the zinc coating near the brazing seam can’t be avoided.ZnAl-alloy filler wires, on the contrary, allow to achieve low joining temperatures similar to the melting point of zinc in the region between 400-500°C and offer new possibilities in order to join zinc coated steels without damaging of the zinc layer. In this paper investigations regarding the set of different ZnAl-alloys (Zn till ZnAl15) for soldering zinc coated steels are reported. Investigations were performed with an Nd:YAG and with a diode laser and confirm the suitability of these alloys for joining these steel types. Mechanical behaviour is reported as well, as a further confirmation of the successfully set of Zn-Alloys for soldering applications.Metallic components undergo during complete lifetime complex loading conditions, which are a combination of mechanical, environmental as well as corrosive loading. The developments within the ULSAB-project allowed the enhancing of mechanical properties of steels, as for example for DP-and CP steel sorts. The corrosion resistance of high strength steels and of deep drawing steel can be only improved through coating technology, as for example through hot dipping in zinc or electrolytic zinc coating. Thermal joining of zinc coated steel sheets presents some difficulties, as the boiling temperature of zinc (907°C) is lower than the steel melting point. As a result of the eruption like evaporation of zinc, pores in the weld seam are present after solidification and the zinc coating near the weldment is spoilt and has to be reestablished. Furthermore the degassing of zinc influences negatively process conditions, as for example arc turbulences when MSG welding and wear of the electrons when resistance spot weld...

The Influence of Gas Cooling in Context of Wire Arc Additive Manufacturing—A Novel Strategy of Affecting Grain Structure and Size

The continuous building process in additive manufacturing with gas metal arc welding (GMAW) provides the main advantage of a decreased processing time but leads to a high heat input in the built work piece. Especially geometrically small parts are affected by a coarse and constantly growing grain structure throughout the continuous reheating process. A novel approach of influencing the temperature-time regime during the additive manufacturing process is an application of additional cooling gas. Experimental trials with argon, hydrogen and nitrogen were carried out and analyzed by means of thermal imaging, hardness measurement and microscopy. The experimental results showed a significant influence of cooling gases on the temperature during the building process. Hence, grain structure and size can be modeled to a homogeneous microstructure by the composition of the gas.

Influence of Processing Conditions on the Mechanical Properties of Aluminium Overlap Joints: A Case Study

Experimental investigations as well as simulation showed in the past years, that hot cracking of aluminium alloys occurs very easily when welding of overlap joints is performed near the sheet edge, as an inhomogeneous and asymmetric temperature field is present, which causes a different shrinkage and expansion behaviour near the weld seam. In order to avoid these phenomena, welding at a distance of at least 8–12 mm is performed in industrial application. This strategy turns to be profitable in order to avoid hot cracks, nevertheless weight saving cannot be achieved successfully. In the reported investigations welding was performed at distances from the edge below 3 mm with the aim to avoid cracking in the weld seam and to achieve a weight reduction. “Centre-line” hot cracking could be process sure avoided at distances lower than 2, 5 mm, as the weld seam turns from a line weld to a flare-bevel-groove weld. Excellent optic appearance was detected when welding at 2 mm distance from the edge at 3 m/min. A displacement of ± 0,5 mm did not lead to a lower static strength of joint. Gap bridging of 0,5 mm without collapsing of mechanical properties is possible. Fatigue testing shows that the position of the weld in the region 0–2,5 mm distance from the edge has no influence on the fatigue strength. Nevertheless failure behaviour varies as crack initiation and growth shift from the transition area to the “root”, where controlling cannot be easily performed.

Effect of local preheating during ultrasonic welding of Al-Cu joints on strand compaction and bond formation

Ultrasonic metal welding represents a well-suited technology for various applications, especially regarding joining of dissimilar materials or sensitive components. For electrical applications, joining stranded wire to contact elements represents a particular challenge, as both the compaction of the strand and the bond towards the terminal have to be realised by the ultrasonic process. This investigation focusses on the influence of local preheating on the joint quality of EN AW 1070 stranded wire to NiP plated EN CW004A terminals, especially regarding strand compaction and bond formation at the interface. The objective of locally preheating the work piece on the sonotrode averted side is to adjust the heat input gradient over the strand cross section and therefore facilitate the deformation of the wires close to the interface. Thus, the compaction process of the strand can be accelerated, vibration damping losses throughout the strand can be reduced and more welding power/energy is available for the bond formation at the interface. The results show a significant increase in failure load, accompanied by a reduced scattering of the values. Moreover, the compaction of the strand and the contact area towards the terminal could also be enhanced by the applied strategy.

Investigations of surface processing of functional ceramics applying ultrashort laser pulses

The development of ultrashort pulse lasers has enabled many new process technologies in the past few years. The nonlinear absorption caused by high peak intensities and the nonthermal ablation are two of the most attractive benefits of pulse durations in the pico- and femtosecond regime, which allow for the processing of a wide variety of materials. Even dielectric, brittle-hard substrates can be processed precisely and gently without cracking or inducing stresses. For this reason, the technology is particularly suited to open up new processing possibilities in the field of functional ceramics. These materials offer many opportunities to set up complex microsystems and multisensor systems. However, the options to structure and shape functional ceramics were limited in the past and had to be solved by elaborate mechanical processes so far. By means of ultrashort pulse laser processing, new applications in the fields of precise shape formation and microstructuring of functional ceramics become accessible. In order to reveal and optimize the processes occurring during surface ablation, investigations with different laser systems have been executed and evaluated by applying various characterization techniques. The results show how the properties of the bulk material and the process parameters such as pulse energy, wavelength, and pulse overlap influence the removal rate as well as the material characteristics, for instance, roughness and morphology. Thereby, the attention is focused on the dependence of the process on the pulse duration. In contrast to the homogenous surface profile that is created during picosecond ablation, the femtosecond process exhibits material modifications in terms of melting patterns. This effect is strongly dependent on the pulse duration, the fluence, and the pulse overlap. It leads to an increase in roughness, which affects the precise material removal. Nevertheless, the investigations also show that the material melting can be utilized to achieve a smoothing effect of the surface if the parameters are well adjusted. The experimental investigations result in optimized process strategies to realize user-defined aims like high ablation rates, high accuracy, or low damage.The development of ultrashort pulse lasers has enabled many new process technologies in the past few years. The nonlinear absorption caused by high peak intensities and the nonthermal ablation are two of the most attractive benefits of pulse durations in the pico- and femtosecond regime, which allow for the processing of a wide variety of materials. Even dielectric, brittle-hard substrates can be processed precisely and gently without cracking or inducing stresses. For this reason, the technology is particularly suited to open up new processing possibilities in the field of functional ceramics. These materials offer many opportunities to set up complex microsystems and multisensor systems. However, the options to structure and shape functional ceramics were limited in the past and had to be solved by elaborate mechanical processes so far. By means of ultrashort pulse laser processing, new appl...

Advances and Potentials in Friction Stir Welding of Aluminum Alloys

Within the last decade, Friction Stir Welding (FSW) has increasingly been gaining relevance for joining nonferrous metals, especially aluminum alloys. Possible applications range from the aerospace and automotive sector up to manufacturing electrical components. Compared to conventional fusion welding processes, FSW offers numerous advantages, as it for example does not require shielding gas or filler material. However, FSW is still not applied or taken into account during the product development process in proportion to its potential. This is mainly caused by the lack of data in order to evaluate the process economically and differentiate it to other processes like arc and laser welding, also regarding technological factors. Therefore, this investigation focusses on the possibilities and limits when joining wrought and cast aluminum alloys, like EN AW-6082 T6, EN AW-7075 T651 and AlSi11Mg0,3, respectively, by FSW compared to MIG. The weld quality of the samples is characterized by tensile testing, hardness measurements and microstructure analysis. Furthermore, an approach to reduce the process forces by using FSW tools with reduced diameters and respectively adjusted process parameters is presented.